The present invention pertains to an ultrasonic atomizer with an ultrasound converter with frequency-dependent impedance characteristic and with a driver system for generating driver signals, especially a driver current for the ultrasound converter, wherein the driver system is used to excite the ultrasound converter with minimal driver current in a range of its counterresonance frequency and to a process for controlling the driver system in an ultrasonic atomizer.
An ultrasonic atomizer of this type has been known from EP 619 761 B1. The prior-art ultrasonic atomizer is operated at its counterresonance frequency with the goal of reaching a good atomizer output at the lowest possible power supply. For optimal atomization of the liquid, the driver system of the ultrasound converter is designed such that the frequency of the driver current is adjusted in the course of the atomization in order to maintain the operation at an optimal frequency. To do so, the driver current flowing through the ultrasound converter is measured continuously and minimized by frequency variation. The frequency value of the driver current at the end of the atomization process is used as the start value for the next atomization.
The drawback of the prior-art ultrasonic atomizer is that when the ultrasonic atomizer is again put into operation, a frequency that deviates from the original setting may be necessary for good atomization because of thermal effects or a change of the liquid to be atomized. If the atomizer frequency is set unfavorably, it is also possible that the ultrasound converter will not perform any vibrations and atomization of the liquid will not take place at all.
The primary object of the present invention is to provide an improved ultrasonic atomizer such that the liquid to be atomized will be atomized optimally with simple handling of the device and to provide a process for controlling the ultrasonic atomizer.
According to the invention, an ultrasonic atomizer with a ultrasound converter with frequency-dependent impedance characteristic and with a driver system for generating driver signals, particularly a driver current for the ultrasound converter. The ultrasound converter is connected such that it receives the driver signals and performs vibrations in a liquid to be atomized. The driver system is used to excite the ultrasound converter with minimal driver current in a range of its counterresonance frequency. The driver system is connected such that a frequency set value with minimal driver current is sought immediately after putting the ultrasonic atomizer into operation by varying the frequency of the driver signal.
According to another aspect of the invention, a process is provided for controlling a driver system in an ultrasonic atomizer, which generates driver signals, especially driver currents, for an ultrasound converter with frequency-dependent impedance characteristic. The ultrasound converter is connected such that it receives the driver signals and performs vibrations in a liquid to be atomized. The driver system is used to excite the ultrasound converter in a range of its counterresonance frequency with minimal driver current. A frequency set value with minimal driver current is sought immediately after putting into operation by varying the frequency of the driver signal.
The advantage of the present invention is essentially that the frequency of the driver current is changed in the region of the counterresonance frequency immediately after putting the ultrasonic atomizer into operation in order to seek the minimum of the driver current. The frequency at which the driver current assumes its minimum is used as the frequency set value for the driver current. Since previous set values are not used, but a favorable frequency is determined for the atomization immediately after putting into operation, the atomizer output can be fully exploited from the beginning.
It is particularly advantageous to seek not only the minimum of the driver current as a criterion for the frequency of the driver current to be set, but to also consider its waviness. The waviness of the driver current is correlated with the quality of the atomization, and good atomization is achieved in case of high waviness of the driver current. Since it may happen that a plurality of minima of the driver current will occur in the frequency range being investigated, the minimum of the driver current at which the highest waviness of the current occurs is selected for the atomization. The waviness of the current is defined as the variation of the driver current around a mean value. The measured waviness signal is determined as the sum of the differences of consecutive measured current values. About 10 to 100 measured current values are evaluated for this. It is advantageous in this connection to preset limit values for both the amplitude of the driver current and the measured waviness signal. The operating conditions for the ultrasonic atomizer are optimal if the driver current is below a predetermined, first limit value and the measured waviness signal exceeds a predetermined, second limit value, because good fountain formation, which leads to a fine, floating aerosol, becomes established in the liquid being atomized at a high measured waviness signal of the driver current.
It is especially advantageous to change the frequency of the driver current in different increments. It is useful to increase the frequency with a first, large frequency increment and to measure the driver current beginning from a start value below the counterresonance frequency to an end value above the counterresonance frequency. Since there is a bandwidth of about 200 kHz between the start value and the end value, the first frequency increment is about 10 kHz to 30 kHz.
The frequency variation with a large increment offers the advantage that longer operation at high driver current values is avoided. The measurement takes place such that a corresponding driver current measured value is determined for each frequency and a minimal, first driver current is determined by comparison with a previous driver current measured value. A frequency variation is then performed around the first minimal driver current with a second frequency increment, and a second minimal driver current is determined in the same manner. The frequency belonging to the second minimal driver current is used as a frequency set value for the ultrasound converter. The frequency variation around the first minimal driver current is carried out with a span of 20 kHz downward and upward with an increment of 1 kHz. It is also possible to begin with a start value above the counterresonance frequency and to perform the frequency variation to an end value below the counterresonance frequency.
In processing the driver current measured values, it is necessary to measure each current value several times, e.g., a hundred times, and to form mean values, because errors could otherwise occur due to stochastic variations.
The ultrasound converter is directly connected to a storage tank accommodating the liquid to be atomized. The electric connection between the ultrasound converter and the driver system is advantageously effected via contact surfaces and the contact tongues touching the contact surfaces, the contact surfaces being arranged on the ultrasound converter and the contact tongues on the driver system. However, it is also possible, the other way around, to provide the ultrasound converter with contact tongues and to arrange the contact surfaces on the driver system. Due to the electrical connection between the driver system and the ultrasound converter proposed according to the present invention, it is possible to separate the two parts in a simple manner.
The ultrasound converter is advantageously fastened to the storage tank by means of an elastic bonded joint. On the one hand, a liquid-tight connection is established by the elastic bonded joint between the two components, and, on the other hand, different thermal expansions, which occur, e.g., during autoclaving, can be compensated. To center the storage tank in relation to the driver system and consequently to align the contact tongues with the contact surfaces, the bottom part of the storage tank, at which the ultrasound converter is located, is designed such that it can be plugged into a mount located on the driver system. The mount is advantageously connected to the supply unit accommodating the driver system in one piece.
An atomizer housing, into which the storage tank can be pushed together with the ultrasound converter, is advantageously provided, and the atomizer housing is closed on the underside with a supply unit, which contains the driver system. The atomizer housing and the supply unit are held together by means of a bayonet catch. The connection can be released by rotating the atomizer housing by a small amount in relation to the supply unit in order to remove the storage tank and the ultrasound converter. On its top side, the atomizer housing has connections for breathing gas tubes, with which breathing gas is introduced into the atomizer housing, on the one hand, and breathing gas enriched with the atomized liquid is sent to a user, on the other hand.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.